Generally, heat exchangers refer to devices transferring heat from high temperature fluid to low temperature fluid through a heating wall, using a heat medium for heat transfer. The heat exchangers can be classified into a water injection type heat exchanger, a double pipe type heat exchanger, a finned tube heat exchanger, and a shell and tube type heat exchanger according to their structure. The double pipe type heat exchanger includes an inner pipe and an outer pipe, in which heat is exchanged between fluid in the inner pipe and fluid between the inner pipe and the outer pipe. Such a double pipe type heat exchanger is mainly used to increase heat efficiency while required to have a reduced size.
FIG. 1 is a view showing an example of a conventional double pipe type heat exchanger, in which a hot water passage is formed inside of the heat exchanger and a heating water passage is formed outside of the heat exchanger.
Heat energy generated from a burner 10 is transferred to a hot water heat exchanging pipe 21 of the heat exchanger 20, and then heated water is discharged through a hot water pipe 32 to the outside of heat exchanger 20. Further, while heating rooms, a circulation pump 51 operates so that heating water receiving heat from hot water in the hot water heat exchanging pipe 21 to a heating water heat exchanging pipe 22 disposed in the hot water heat exchanging pipe 21 is supplied through an outlet 41 of the heating water heat exchanging pipe 22 to a place where heating is required so as to transfer heat thereto.
A reference numeral 11 which is not described above denotes a blower for blowing combustion heat of the burner 10 to the heat exchanger 20, and a reference numeral 31 indicates a cold water pipe in which a cold water is supplied to be heated as hot water. A reference numeral 42 refers to a heating water returning inlet through which the heating water returns after being discharged from the heating water outlet 41 and circulating heating pipes, and a reference numeral 50 denotes an expansion tank for removing air generated in the heating pipe, decreasing pressure which is caused in the pipe by expansion of the heating water according to increasing the temperature of the heating water, and regularly maintaining the amount of water in a boiler.
If a required heating temperature is high, the temperature of an outer heat exchanging pipe 21 becomes higher than the temperature of an inner heat exchanging pipe 22. When hot water is used during heating rooms at a relatively high temperature, the hot water is discharged from the outer heat exchanging pipe 21 of the heat exchanger 20. When high temperature hot water is discharged from the heat exchanger 20, there is a problem in that a user is exposed to a danger in scald.